This invention relates to the processing of expandable thermoplastic polymeric particles for use in the production of low density molded thermoplastic articles and more particularly to the production of partially expanded thermoplastic polymer particles having a uniform size and density.
Because of their excellent insulating and buoyancy properties and low cost expanded thermoplastic materials are widely used in the manufacture of such articles as ice chests, hot beverage drinking cups, floats, etc. These items are generally manufactured by expanding thermoplastic particles containing a blowing agent in a closed mold having the configuration of the finished article. The particles are heated to a temperature above the softening point of the thermoplastic material. When the thermoplastic material softens the blowing agent, now in the form of a gas under pressure, exerts a force on the inside portions of the particles causing them to expand outwardly.
Due to the high ratio of expansion the blowing agent containing thermoplastic particles are usually partially expanded or "pre-expanded" to a density of about 1/30th to 1/40th of their original density. The pre-expanded particles are then usually aged to permit air to infuse into the pre-expanded particles to replace blowing agent which has diffused out of the particles leaving a partial vacuum in the interior portions of the particles. After a suitable aging period the partially expanded particles are blown into a mold where they further expand to fill the mold whereupon they fuse together to form the finished article. The density of molded articles is much more uniform when the expandable particles are pre-expanded than when the beads are directly injected into the mold for full expansion therein. The pre-expansion of the expandable thermoplastic particles has been accomplished by several methods with various degrees of success. In accordance with one well known practice expandable thermoplastic particles containing, as a blowing agent, a low boiling hydrocarbon are heated by means of a hot gas, generally air, to effect the preexpansion. This is conveniently accomplished by blowing the hot air into a chamber containing the particles and equipped with means for providing agitation thereof whereby they are heated sufficiently to effect the pre-expansion. Since the hot air has a relatively low diffusion rate through the polymer material very little of the air is able to penetrate the particles; therefore most of the heating of the particles take place via conduction from the outside surfaces to the interiors of the particles. This is a very inefficient process due to the excellent insulating properties of the thermoplastic material and the density of the product produced is relatively high.
In another process steam is used as the heating medium. It has been determined that steam penetrates and diffuses into the thermoplastic material such as polystyrene much more rapidly than does air, for example, in the order of about one thousand times faster. (Modern Plastics, January 1965, p. 173). Accordingly, steam is much more efficient than hot air for heating expandable thermoplastic particles because the thermoplastic particles and the gases contained therein can be heated from the inside of the particles as well as from the outside by convection. While the use of steam results in a product with a relatively low density it presents a serious disadvantage in that the pre-expanded particles become wet from condensing steam and have a great tendency to agglomerate.
Accordingly, when steam is used as a heating medium the pre-expanded beads must be dried for long periods of time, for example, for as long as 24 hours or more, in order to render the beads free flowing.